A magneto-optical disk, a variety of magneto-optical recording media, is already commercialized as an external memory for a computer. This example demonstrates that the magneto-optical recording medium, using light for reproduction, can realize a larger recording capacity than a floppy disk and a hard disk, using a magnetic head.
Recording density of the magneto-optical disk is limited by the size of a light beam spot on the magneto-optical disk. Specifically, if the recording bit diameter or the recording bit interval is too small in comparison to the light beam spot, the light beam spot covers more than one bit, and thereby cannot separate recording bits for reproduction. Shortening the wavelength of the laser beam is an effective way of reducing the size of the light beam spot in order to improve the recording density. Nevertheless, semiconductor laser devices currently on the market cannot produce a wavelength shorter than 680 nm, and a semiconductor laser device capable of producing a shorter wavelength is still in a development stage. Therefore, it is difficult to further improve the recording density of the magneto-optical disk with semiconductor laser devices currently on the market which are only capable of producing long wavelengths.
On the other hand, for instance, Journal of The Magnetics Society of Japan, Vol. 19, Supplement, No. S1 (1995), page 421-424 shows a method to improve recording density by improving reproduction resolution with a magneto-optical recording medium composed of two magnetostatically coupled magnetic films and temperature distribution in a light beam spot, i.e., a magnetically induced super resolution (MSR) technique using magnetostatic coupling.
A signal detected with the above-mentioned MSR technique using magnetostatic coupling has good signal quality (CNR) with a short mark length and therefore is capable of improving the recording density, compared to a method not employing the MSR technique. However, the shortest mark length possible for the method is 0.5 .mu.m. Therefore, the method still has a problem of low CNR when the mark length is shortened to, for example, 0.25 .mu.m in order to further improve the recording density.